(1) Field of the Invention
The present invention relates to a solid state image sensor device, and more particularly to a wiring structure for a peripheral circuit of the solid state image sensor device.
(2) Description of the Related Art
A wiring film provided for a peripheral circuit of a conventional solid state image sensor is shown in sectional view in FIG. 1. The wiring film of the peripheral circuit is in the form of an aluminum wiring film 6 which is provided on an insulating film 1 on a semiconductor substrate 14.
FIG. 2 shows in sectional view a unit picture element of the conventional solid state image sensor, which is constituted by a photoelectric conversion element 8, a transfer gate 9, a vertical register 10, a channel stopper 11, a vertical transfer electrode 12, and an aluminum light-shielding film 13. The aluminum shielding film 13 is provided on tile vertical transfer electrode 12 since, in an effective light detection region, a light beam incident on the vertical register 10 appears in a reproduced image as a false signal called a smear. Usually, the aluminum shielding film 13 is formed, for simplicity of the fabrication, by use of aluminum (Al) in the same fabrication process step as that for wiring a drive circuit for driving the transfer gate 9, the vertical register 10, and a horizontal register, and for wiring a peripheral circuit such as an output processing circuit for processing an output from a signal electron detection portion of the device.
Where the number of picture elements in the solid state image sensor is increased in order to improve the resolution thereof, the frequency for reading out the signal electrons becomes higher accordingly. In order to cause a signal transfer clock pulse of a high-frequency to propagate in such a way that its waveform does not become dull, it is necessary for the resistance of the wiring for the peripheral circuit go be sufficiently lower. Furthermore, with an increase in the chip size due to the increase in the number of picture elements, the length of the aluminum wiring for the peripheral circuit becomes longer, which leads to an increase in the wiring resistance. In order to lower the wiring resistance of the aluminum wiring of the peripheral circuit, one may attempt either to increase the thickness of the aluminum film or the width thereof. When the thickness of the aluminum film is increased, the thickness of the aluminum shielding film which is formed in the same process will also be increased so that, due to deterioration of processing precision, there arise sensitivity irregularities caused by variations in apertures in various picture elements. So, one may then consider forming, for example, the wiring film for the peripheral circuit and the aluminum shielding film for the unit picture element separately from each other with only the wiring film for the peripheral circuit being made thicker.
For the wiring film of the peripheral circuit and the aluminum shielding film to be formed separately in order to lower the resistance of the aluminum wiring film of the peripheral circuit without increasing the thickness of the aluminum shielding film, the aluminum etching in the form of dry-etching may usually be performed. However, in the case where the aluminum film is thick, it will be necessary to prolong the time for etching (i.e., over-etching) so as not to leave aluminum in place. The over-etching may cause an active region to be damaged and a dark current to be increased.
As to the attempt to lower the resistance of the wiring by increasing the width of the aluminum film, this is not readily made practicable because of the restrictions from chip sizes or wiring layouts.